There are many access methods for radio communication systems where plural mobile stations share at least one common radio channel. According to one known method mobile stations do not transmit messages on the common radio channel until they have received a transmission invitation message authorizing them to transmit such messages. A base station ready for receiving certain kinds of messages from at least some mobile stations transmit a transmission invitation message comprising information on a number of time slots and slot length of transmission time slots on the common channel following the transmission invitation message. The transmission invitation message may also comprise information on which mobiles are authorized to transmit in succeeding time slots and what kind of messages and maximum length of messages that may be transmitted.
Any mobile station having a message to be transmitted on the common radio channel selects time slot for transmission after receiving an invitation message from the base station authorizing the mobile station to transmit the message. Plural mobile stations may be authorized simultaneously and each mobile station may select a time slot and a transmit its message independently of any other authorized mobile station. Accordingly, sometimes two or more mobile stations may transmit their messages in the same time slot whereby the base station may be unable to receive and understand any of the simultaneous messages. The base station transmits an acknowledgement message to any mobile station after receiving a message from that mobile station in a transmission time slot on the common radio channel according to an invitation message from the base station. The acknowledgement message is transmitted from the base station prior to transmitting a new transmission invitation message concerning the common radio channel.
Any authorized mobile station, having transmitted a message to the base station on the common radio channel but having not received an acknowledgement message from the base station prior to receiving a new transmission invitation message from the base station concerning the common radio channel, selects a new transmission time slot according to the new transmission invitation message and transmits a repetition of its message. The new transmission slot for the repetition is choosen among a group of random slots.
There may be various traffic and performance requirements on a radio communication system with base stations and plural mobile stations, in particular when the system is intended to cover a wide geographical area. Considering strong demands for radio frequencies and the increasing shortage of available frequencies, local, federal or governmental authorities granting use of frequencies may interested in efficient frequency use. A network operator responsive for base stations and their interconnections including exchanges and the overall system performance may be interested in a system which is profitable without excessive base station equipment and affords good service to subscribers. A network operator may be interested in a system using his equipment in an efficient way with easy to expand to suit a growing number of subscribers. A subscriber may be interested in a system offering high accessability and short response and mean waiting times. A subscriber may also be interested in reliable equipment to reasonable price and low subscription and communication rates.
Sometimes the interests of authorities, operators and subscribers are contradictory. The best way to meet operator requirements and compromises between authorities, operators and subscribers interests may change with time due to changing traffic load. Accordingly, it is important to be able to change at least some system parameters in order to optimize a radio communication system to traffic and performance requirements. In a system of the kind described above, there are several system parameters that can be changed, e.g. number of time slots after a particular invitation message, time slot length, the maximum length of a mobile station message, kinds or number of mobiles authorized to transmit after a particular invitation message, kinds of messages allowed to be transmitted after a particular invitation message and number of common channels. To be able to control system parameters in the right way, it is important to know traffic load and prevailing system performance with sufficient accuracy.
One method of adapting a communication system with base and mobile stations sharing a common channel is described in U.S. Pat. No. 4,398,289. According to this method, the base station classifies each time slot into one of three categories. A first category of time slots, called correct slots, are time slots in which the base station receives an understandable message from a mobile station. A second category of time slots, called empty slots, are time slots in which there apparently is no transmission of any message from any mobile station. A third category of time slots, called mutilated slots, are time slots where apparently at least one mobile station is transmitting a message but the base station cannot understand the message due to noise or interference from at least one other mobile station transmitting in the same time slot. The number of correct slots, the number of empty slots and the number of mutilated slots are counted for each invitation message. In the base station, the number of time slots of the next invitation message is adapted on the basis of the number of correct, empty and mutilated time slots of the previous invitation message and taking into consideration the conditional probabilities of the occurrence of correct empty and mutilated time slots, given the number of the mobile stations transmitting in a time slot. Another method of adapting a communication system with base and mobile stations sharing a common channel is described in U.S. Pat. No. 4,672,608. According to this method the system may operate in one of four modes, namely, light traffic mode, dynmic frame length mode, hybrid mode and polling mode. The invitation message from the base station comprises an operational code indicating the selected mode of operation. The value of the operational code and thereby the particular one of the modes is determined in response to an estimate of the prevailing traffic conditions.
Various parameters for adapting a radio communication system with base stations and plural mobile stations to traffic and performance requirements are available in a system called MOBITEX. In the available documentation on the MOBITEX, system control of several system parameters are suggested for the purpose of meeting various traffic loads and performance requirements, e.g. number of time slots, slot length, number of channels, maxim message length and selective authorization. There is, however no suggestion regarding how to estimate prevailing traffic load or prevailing system performance, e.g. access time or response time or mean waiting time.